Unlike other displays such as the CRT (Cathode Ray Tube), PDP (Plasma Display Panel), or EL (Electro Luminescence), in liquid crystal display devices, the liquid crystal itself does not emit light, but displays letters or images by regulating the quantity of light transmitted from a specific light source.
Conventional liquid crystal display devices (hereinafter referred to as "LCDs") can be roughly divided into transmission-type LCDs and reflection-type LCDs. Transmission-type LCDs include a fluorescent tube or surface luminescent light source such as an EL provided on the back of the liquid crystal cell as a light source (back-light).
On the other hand, reflection-type LCDs, since they perform display using surrounding light, do not require a back-light, and thus have the advantage of low power consumption. Further, in very bright areas such as in direct sunlight, whereas the display of light-emitting displays and transmission-type LCDs becomes nearly impossible to see, that of reflection-type LCDs becomes more clearly visible. For this reason, reflection-type LCDs are applied in devices such as portable information terminals and mobile computers, for which demand has grown in recent years.
However, reflection-type LCDs have the following problems. Namely, since reflection-type LCDs use surrounding light, the brightness of display is highly dependent on the surrounding environment, and, in darkness, such as at night, there are cases when the display is not visible at all. This problem is particularly serious with reflection-type LCDs which use a color filter for color display or which use a polarizing plate, and auxiliary illumination is needed to provide against cases when surrounding light is insufficient.
However, since reflection-type LCDs are provided with a reflective plate on the back of the liquid crystal cell, they cannot use a back-light like that of transmission-type LCDs. A device called a "semi-transmission-type LCD" has been proposed, but since its display characteristics, being midway between transmission-type and reflection-type, are neither here nor there, practical application of this device is expected to be difficult.
Therefore, as auxiliary illumination for reflection-type LCDs when surrounding light is insufficient, a front-light system, for mounting on the front of the liquid crystal cell, has been proposed. Generally, such front-light systems have been made up of a light-conducting body and a light source provided at the side of the light-conducting body. Light projected by the light source from the side of the light-conducting body travels through the interior of the light-conducting body, and is reflected toward the liquid crystal cell by forms provided on the surface of the light-conducting body. As it passes through the liquid crystal cell, the projected light is modulated in accordance with the display information, and, being reflected by the reflective plate provided on the back of the liquid crystal cell, passes again through the light-conducting body toward the viewer. By this means, the viewer is enabled to see the display even when the surrounding light is insufficient.
Front-light systems of this type are disclosed, for example, in Japanese Unexamined Patent Publication No. 5-158034/1993 (Tokukaihei 5-158034) and in SID DIGEST (1995), p. 375.
The following will explain in brief the driving principle of the front-light system disclosed in SID DIGEST (1995), p. 375 with reference to FIG. 26. This front light system is provided with a light-conducting body 104, which has an interface 101 made up of flat portions 101a and inclined portions 101b, one side of the light-conducting body 104 being a light-entry surface 105, through which light from a light source 106 enters the light-conducting body 104. In other words, the light source 106 is provided in a position opposite the light-entry surface 105 of the light-conducting body 104.
Some of the light from the light source 106 entering the light-conducting body 104 from the light-entry surface 105 travels straight, and some of it is projected onto interfaces 101 and 108 between the light-conducting body 104 and the surrounding medium. At this time, if the medium surrounding the light-conducting body 104 is air, and if the refractive index of the light-conducting body 104 is around 1.5, then, according to Snell's law (Equation 1), light with an angle of incidence at the interfaces 101 and 108 of approximately 41.8.degree. or more will be totally reflected. EQU n.sub.1 .cndot.sin .theta..sub.1 =n.sub.2 .cndot.sin .theta..sub.2 EQU .theta..sub.c =arcsin(n.sub.2 /n.sub.1) (Equation 1)
Here,
n.sub.1 is the refractive index of the first medium (here, the light-conducting body 104); PA1 n.sub.2 is the refractive index of the second medium (here, air); PA1 .theta..sub.1 is the angle of incidence from the light-conducting body 104 at the interface 101; PA1 .theta..sub.2 is the angle of light exiting from the interface 101 to the second medium; and PA1 .theta..sub.c is the critical angle.
Of the light projected onto the interfaces 101 and 108, the portion of light which is totally reflected from the inclined portions 101b (which are reflective surfaces) and the portion of light which, after being reflected from the interface 108, is reflected from the inclined portions 101b, are projected into a liquid crystal cell 110. Light projected into the liquid crystal cell 110, after being modulated by a liquid crystal layer (not shown), is reflected from a reflective plate 111 provided on the back of the liquid crystal cell 110, is projected once again into the light-conducting body 104, and passes through the flat portions 101a toward the viewer 109.
Light from the light source 106 entering through the light-entry surface 105 which is projected, not onto the inclined portions 101b, but onto the flat portions 101a, continues being transmitted and reflected between the interfaces 101 and 108 until it reaches an inclined portion 101b. Incidentally, the inclined portions 101b are provided so that their area, in comparison with the area of the flat portions 101a, is sufficiently small when viewed by the viewer.
The foregoing front-light system has the following problems.
(1) As shown in FIG. 27, light which does not reach an inclined portion 101b even after repeated reflections, and light which enters the light-entry surface 105 substantially perpendicularly, exit the light-conducting body 104 from a surface 107 opposite the light-entry surface 105 as light 114, and cannot be used in display. This problem is more marked the smaller the panel is, and with the sizes typically used in portable information terminals (5 in. to 6 in. diagonal), most of the light from the light source exits the light-conducting body, and thus the efficiency of light use is very poor.
(2) The form of the interface 101, which is made up of inclined portions 101b and flat portions 101a, is similar to that of a prism sheet with the peaks of the prisms flattened. Thus, as shown in FIG. 27, surrounding light 115 is easily reflected back toward the viewer 109, which leads to impairment of display quality.
Since most conventional front-light systems share these problems, improvement of the efficiency of use of light from the light-source is needed.